Climbing rucksacks are narrow so the don't drag in the wind or buffet about as much but the long straps can dangerous when your sat so close to spiny things.
Knox do a nice looking rectangular one and Givi do a roll top one like a dri-bag, I like the roll top ones because water can't get forced into them.
Cycle shops usually have good ones.
Motorcycle specific ones tend to be heavy/robust compared to "outdoor" ones.
or get a Burgman 400, the storage is mahoosive
